In the manufacture of pigmented finishes one rarely if ever achieves a satisfactory color match versus a color standard without an adjustment process known as shading. Shading usually involves a relatively minor but critical manipulation of the formula pigment composition, correcting for the cumulative effects of manufacturing variables on pigment dispersions.
Traditionally, the shading process has been carried out by highly skilled and trained personnel who require extensive on-the-job experience to achieve proficiency in their craft. Since visual shading at best is an art, effective administration of the process was difficult.
In more recent years, such visual shading has been supplemented by the use of apparatuses for instrumentally characterizing a paint or pigment composition. Colorimeters and spectrophotometers are well-known in the art and are used to measure certain optical properties of various paint films which have been coated over test panels. A typical spectrophotometer provides for the measurement of the amount of light reflected at varying light wavelength in the visible spectrum by a painted panel that is held at a given angle relative to the direction of an incident source of light. The reflectance factor of the paint enables paint chemists to calculate color values by which to characterize various paint colors. For a paint containing no light-reflecting flakes or platelets (i.e., non-metallic paints), the reflectance factor will not vary with the angle of the panel relative to the direction of incident light except at the gloss (specular) angle. Consequently, a single spectrophotometric reading at any specified angle will produce a reflectance value by which to accurately characterize the paint.
However, the paint industry often utilizes light-reflecting flakes in paints (i.e., metallic paints) to obtain pleasing aesthetic effects. Paints containing light-reflecting flakes of such materials as aluminum, bronze, coated mica and the like are characterized by a "two-tone" or "flip-flop" effect whereby the apparent color of the paint changes at different viewing angles. This effect is due to the orientation of the flakes in the paint film. Since the color of such metallic paints will apparently vary as a function of the angle of illumination and viewing, a single spectrophotometric reading is inadequate to accurately characterize the paint. Although measurement studies have shown that visual color differences existing between two metallic paints were detectable at an infinite number of angles, it is obvious that practical reasons preclude the collection of reflectance factors for an infinite number of viewing angles. However, previous studies have also indicated that measurement of the optical properties of a metallic paint at only two specified angles can provide useful characterization. See, for example, U.S. Pat. No. 3,690,771, issued Sept. 12, 1972 to Armstrong, Jr., Edwards, Laird, and Vining, the disclosure of which is herein incorporated by reference.
The present invention relates to the discovery that unexpectedly improved optical characterization of metallic paints results when measurements are taken at three specified angles.